This invention relates to compounds useful as a vehicle for a paste for electronics applications and as a fluxing additive in wire solder fluxes. More particularly, this invention relates to a liquid vehicle for suspending inorganic particles which are used in thick film and solder paste applications. 2. Description of Related Art.
The electronics industry uses particulate-based high-solids pastes in a variety of applications to ultimately provide conductive or dielectric capability in an electronic component. These pastes typically contain inorganic particles and an organic medium or vehicle. The inorganic particles provide the electrical functionality in the ultimate application, i.e., insulative or non-conducting inorganic particles form dielectric parts, and conductive inorganic particles, primarily metals, form conductive parts. The organic medium facilitates application and control of the location of the inorganic particles. After application of the paste, the organic medium is removed by heating and the inorganic particles are fused or sintered to form the electrically functional part. To apply the paste to the desired locations in the desired patterns, it is necessary that the paste flow well with the application method employed, such as stencil or screen printing, pressure dispensing, syringe dispensing, etc.
A rheology modifying agent or agents is typically included in the paste formulation for the paste to have the appropriate flow or rheological characteristics. Particularly desirable rheological characteristics of the paste include high viscosity, which precludes separation or settling of the inorganic particulate in the paste, dripping, stringiness, and slumping or sagging; pseudoplasticity, which provides shear thinning for improved printing characteristic; sufficient tackiness to hold components; good transfer through the printing pattern; good print definition; and clean release from the screen or stencil.
Effective amounts of rheology modifying agents, other organic ingredients, and acidic and/or basic activators, are typically so high that the organic ingredients leave a substantial residue on the solder after reflow. For example, rosin is typically present in a solder paste in the amount of 1 to 6% by weight of the paste. This amount of rheology modifying agent leaves a substantial, readily visible and potentially detrimental solder-coating residue.
It is necessary to wash off residues since the residues contain various ionic contaminants and other chemical agents which can cause shorting and product failures over time. Despite the undesirability of these residues, they have been of little concern until recently because chlorofluorocarbon solvents effectively and easily remove the residues. Today, however, considerable efforts are being made to replace such chlorofluorocarbon solvents due to their environmental impact upon the earth's stratosphere. In addition the failure of cleaning agents to completely remove residues from tight, hard-to-reach spots, and from under components in surface mount or fine pitch assembly operations, makes current methods and proposed alternative cleaners less effective.
As a result, there arises a need for a solder paste composition which does not require cleaning of the residues remaining after reflow. To eliminate the need for cleaning, it is critical to formulate pastes in such a manner that after reflow the amount of residue is minimal and innocuous. Residue should be minimal so as to not interfere with electrical probing of the solder joint, adhesion of conformal coatings, as well as for aesthetic reasons. The residue must be innocuous so it will neither cause corrosion nor provide a conductive path for leakage currents between adjacent conductors.
Similarly, in electronic thick film pastes, the amount of organic ingredients should be minimized. When present in the final product, such ingredients and residues thereof, are undesirable contaminates that deleteriously effect the electrical performance of the part.